In many applications for radio transmitters it is desirable to minimize power consumption whilst achieving sufficient signal fidelity or linearity. For instance, the power consumption is minimized by using an envelope tracking (ET) technique of the power supply whereas the desired signal fidelity or linearity is achieved by using digital pre-distortion (DPD) technique. Hence, digital pre-distortion techniques are typically used in radio frequency applications, in particular in radio transmitters.
By using the digital pre-distortion technique, a signal distortion is determined that is caused by a transmitter unit of the radio frequency device that processes the signal, in particular by a unit of the transmitter unit that has an amplifying function. This signal distortion is derived from measured data of the signal output by the radio transmitter unit. This signal distortion is expected to occur repetitive while using the radio transmitter unit. Accordingly, the signal distortion caused by the transmitter unit is characterized by the specific transmitter unit.
After the signal distortion has been determined, the distortion to be expected is inverted and the inverted distortion is applied to the original signal in a complementary sense prior to the transmitter unit, in particular an amplifier unit of the transmitter unit, such that the original signal is pre-distorted by the inverted distortion. Then, the pre-distorted signal passes through the transmitter unit, in particular its amplifier unit, such that the pre-distortion of the signal is cancelled by the distortion that is caused by the transmitter unit appropriately. Accordingly, the result at the output of the transmitter unit is a signal having a high fidelity with regard to the signal input.
Usually, two different techniques are used for digital pre-distortion which are called memoryless digital pre-distortion and memory mitigation digital pre-distortion. Both methods distinguish from each other by the way of predicting the distortion caused by the transmitter unit. The memoryless digital pre-distortion is incapable of compensating memory effects that affect the efficiency of the digital pre-distortion. In contrast thereto, memory mitigation digital pre-distortion is able to compensate memory effects. However, memory mitigation digital pre-distortion is very complex such that those systems are not suitable for small radio frequency devices such as a mobile device.
As already mentioned above, the enveloped tracking technology is used for minimizing power consumption of the radio frequency device. However, the enveloped tracking technology provides another signal with respect to the radio frequency signal such that two different signals are transmitted. Thus, a high degree of precision is required ensuring that both signals have the same time of arrival (TOA) at the transmitter unit. Misalignments in the time of arrival of both signals cause a degraded linearity and increased memory effects wherein these memory effects result in degraded linearity.
Accordingly, highest fidelity of the signal is achieved by determining the predicted pre-distortion precisely and accurately while the time of arrival difference is minimized.
However, there is a need for a pre-distortion technology being less complex, but effective.